Thursday, January 27, 2011

Day 4- Riding the Ridge

We are at the proverbial "halfway point" to Sunday. With each day, we have seen an increase chance in precipitation and not much change in the way of high/low temperature. We are on the cusp of getting much more data starting tomorrow. That is when multiple models start their Sunday runs. For the past 4 days I have been using extended GFS (Global Forecast System) models to forecast. GFS, NAM (North American Model) and more GFSx models will be available for use. Also, the University of Washington has a very good model system set up that uses NAM and WRF (Weather Research and Forecasting) model that specifically focus on the Pacific Northwest. That will help as well. So with more data means more information to sift through. This is where the fun really begins. But as for today's update:

Yesterday we saw a nice batch of moisture being pushed our way with the help from high pressure to our south and low pressure to our north. Today that moisture has been pushed back into the flow of the low pressure system. That moves the precipitation to the north and removes a good chance of rain for the majority of the day Sunday. Due to this, I have removed the showers for most of Sunday.
By the way, check out the Pacific weather charts as of today. You can see that area of low pressure that will eventually make its way into the Gulf of Alaska. It's sitting out in the middle of the Pacific. Pretty neat that we can see this coming from so far away!

I am adding a new map to the discussion today. This is the 1000mb map and is essentially a surface map. Many locations around the state would call 1000mb "ground level". What I am looking for in this map will be re-enforced all the way up to the 500mb map today, more on this in a bit.
I have highlighted a few wind vectors. Wind vectors on this map indicate direction and wind speed. The longer the arrow, the faster the winds. Notice how the winds around Oregon spread apart from one another. This indicates that wind at the 1000mb level are diverging away. This is cause when air sinks from the upper levels. When we have air sinking, once it hits the ground the only thing it can do is spread out. Sinking air also denotes stable conditions and high pressure. This is another sign that our rain chance for Sunday is diminishing at this point.


High pressure off the California coast is still the dominant feature at 850mb. The low up to the north weakened on the maps from yesterday. There appears to be little to no warm air advection on today's map. That favors more stable conditions.
That pesky shortwave makes a return in today's map but it doesn't impact our forecast. What does impact the forecast is the ridge that has been re-established. This ridge is just off the coast. I would not be surprised to see this ridge move a bit more between now and 4pm Sunday. This ridge is re-enforced at the 1000mb level. Remember the divergence of wind on that map? It is a result of this ridging.


The jet streak we saw yesterday that was set to help with the precipitation has now left. It has sagged to the south along with that shortwave energy.
All in all, day 4 has brought an improvement in the forecast. I would say the only threat of rain on Sunday will be limited to the early morning hours. The first four posts have been strictly conditions updates. Tomorrow I will get into some number forecasting as various models and text models will begin to include Sunday. As of today, my initial numbers have held up pretty well!

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